I've made a soundset for Freeciv (freeciv.org) using exclusively samples from the freesound project (I will put a link to the soundset+forum posting in a later post since I'm being spam blocked.) I have two questions about it.

First, is this ok? I'm using the samples, most of them with very few changes. The attributions are all in place inside the documentation of the soundset, written according to the legal FAQ page of this site. Is this ok to distribute?

Secondly, I'd also like to be able to distribute the soundset with the game. Unfortunately the game itself is GPLv2, which is -- apparently -- not compatible with a licence which prohibits commercial re-use. Would I be working in vain if I were to try to contact each sample creator and ask them to relicense the sample as GPLv2?

Thanks. I'm a big fan of free software and of Creative Commons. I'd like to do things by the book and also create something everyone can enjoy!